It is known to ensure the safety of an elevator's operation by means of a safety chain. The latter includes an electric line fed by a power source, where all the landing door locks are installed in series the line. When all the locks are secured and in good operating condition, the safety chain is complete and a current passes through it. The chain governs a control device which powers the driving motor of the elevator cabin.
By contrast the safety chain is incomplete if even one of the locks is defective: the motor is not powered and the cabin remains in the stopped position. It is then necessary to intervene to repair the defect. If the latter is apparent it is easy to discover and repair it, but in the contrary case, for example when all the landing doors are properly closed and the cabin does not operate in spite of this, it is necessary to check all the landing doors individually, as well as all the locks. At best this work can be performed fairly easily and without much effort when the installation only has one or two floors, but in the case of a building with a large number of floors, the search of the defect is tedious, lengthy and costly.